1. Field of the Invention
The present invention relates to a radio-frequency power amplification circuit and, more particularly, to stabilization circuit for stabilizing a gain of a radio-frequency power amplification circuit.

2. Description of the Background Art
In designing a radio-frequency power amplification circuit (hereinafter abbreviated as “amplification circuit”) that is used in a microwave band, a submillimeter wave band, or a millimeter wave band or an MMIC (monolithic microwave integrated circuit), an optimum type of active element, gate width thereof, bias conditions, etc. are selected in accordance with a required frequency, gain, output power, etc.
To attain high performance and low cost of the entire microwave/millimeter wave system (entire chip set) including an oscillator, a mixer, and a frequency multiplier, a type of active element, its gate width, bias conditions, etc. may not necessarily be set so as to be most suitable for an amplification circuit.
For example, in designing an intermediate-frequency amplification circuit to be used in a millimeter wave system, there may occur a case that it is necessary to use an active element that can operate in an unnecessarily high frequency range. This is to give priority to conditions that are most suitable for other devices because this amplification circuit is not a device that directly affects the performance of the millimeter wave system. In this case, it is necessary for a stabilization circuit to attain stabilization in a wide frequency range covering ranges above and below a design frequency band (hereinafter abbreviated as “design band”).
FIG. 4 is a circuit diagram of a conventional radio-frequency power amplification circuit.
In FIG. 4, reference numeral 10 denotes a field-effect transistor (hereinafter abbreviated as “transistor”) used as an active element and numeral 40 denotes a stabilization circuit that is provided upstream of the transistor 10. The stabilization circuit 40 includes a resistance element 42, one end of which is connected to a gate terminal of the transistor 10 and an input terminal 1, a short stub 43, one end of which is connected to the other end of the resistance element 42, and a capacitance element 44, one end of which is connected to the other end of the short stub 43, and the other end of which is grounded.
A drain terminal of the transistor 10 is connected to an output terminal and its source terminal is grounded.
FIG. 5 is a graph showing frequency characteristics of the gain (MAG, MSG) of the conventional radio-frequency power amplification circuit of FIG. 4.
In FIG. 5, a broken line A indicates a frequency characteristic of the gain (MAG, MSG) of the active element 10 itself and a solid line C indicates a frequency characteristic of the gain (MAG, MSG) of the combination of the active element 10 and the stabilization circuit 40. In FIG. 5, the horizontal axis which represents the frequency has a logarithmic scale.
In the case of the active element 10 itself, as indicated by the broken line A, the gain of the amplification circuit is not stable in a wide frequency range covering ranges above and below a design band; an MSG range (i.e., an unstable range where the stabilization coefficient k<1) is formed there.
On the other hand, where the stabilization circuit 40 is added to the active element 10, the gain of the amplification circuit is stable in a wide frequency range covering ranges above and below the design band as indicated by the solid line C.
However, there remains a problem that when it is intended to stabilize the gain in a frequency range higher than the design band by using the conventional stabilization circuit 40, the gain of the amplification circuit is much lower in the design band as indicated by an arrow in FIG. 5.